This invention relates to apparatuses for out of contact detection of rotations of bodies undergoing rotational displacements, in particular apparatuses for detecting the rotational speed of internal combustion engines for vehicles, which apparatuses are suitable particularly for diesel engines.
Conventional apparatuses for such purposes detect the rotation of a gasoline engine by means of ignition signals therefor or by means of a sensor such as an electromagnetic pick-up which senses the motion of the teeth of an engine cranking gear.
However, the former of the above apparatuses can not be applied to diesel engines which do not utilize spark ignitions, while the latter require boring engines themselves for mounting thereof.
It has been well known that one revolution of a magnetic body having a pair of opposite poles (N and S poles) introduced forcibly by an external magnetic field, will generate one cycle of a change in the magnetic field. We have further found after a variety of experiments that a ferromagnetic body free of external magnetization also exhibits, in one cycle of its revolution, one cycle of a change in the magnetic field due to its inherent magnetization.